This invention relates to a desublimation, especially fractional desublimation, process and apparatus therefor. Desublimation is defined as the direct phase change of a vapor into a solid, i.e., without passing through a liquid phase.
In the production of inorganic and organic solids, sometimes solids are initially obtained in the vapor phase in a non-desublimable carrier gas and are subsequently subjected to fractional desublimation by indirect heat transfer on cooling jacket surfaces or the like. These processes require large cooling surfaces. Because desublimed solids adhere and accumulate on the walls, the resultant heat transfer characteristics of the wall surface are changed in an uncontrollable manner, resulting in a variation in solids quantities and qualities, and a relatively rapid clogging of the apparatus. For this reason, scraping devices are frequently employed in the apparatuses, but such devices are expensive both to purchase and maintain, and when out of service, lead to down-time of the desublimation system.
An alternative desublimation system relies on the direct cooling of the gas-vapor mixture with coolants present in the gaseous phase under the conditions of desublimation. Thus, for example, according to British Pat. No. 1,081,579, water, as a preferred coolant, is introduced under pressure via nozzles into the gas-vapor mixture and vaporized therein. However, in this process, besides the desired solid product, solid by-products are also deposited on the water droplets before the droplets are entirely vaporized, so that fractional desublimation is substantially impeded. When water is employed as the coolant, the process is likewise unsuitable for the desublimation of substances sensitive to water, such as, for example, acid anhydrides. Since the walls of the desublimator are heated on the one hand, and the residence time of the gas-vapor-liquid mixture in the desublimation zone is very long on the other hand, some of the coolant is also heated by the walls, thereby requiring large quantities of coolant for the process. This is especially the case when using cooling fluids having lower heats of evaporation than water or when using gases. In addition gas is introduced at the bottom of the desublimator. Accordingly, this also adds to coolant load as well as increasing the amount of resultant waste gas.
In a process according to German Pat. No. 1,108,663, a carrier gas, freed of product, is recycled via coolers and mixed with the product-containing gas. Simultaneously, liquid product is injected into the mixing chamber. Fractional desublimation is thereby difficult to perform. In French No. 2,082,822, it is suggested to conduct desublimation with air in two series-connected pipes to obtain improved deposition of the solid on the pipe walls and to gain a purer solid. This process requires extraordinary large cooling surfaces for an industrial scale installation, and in any case, the pipes would be relatively rapidly plugged by the deposited solids.
A similar process is described in German Pat. No. 2,617,595 wherein cooling of the gas-vapor mixture is conducted with both cooling gases and cooled pipes equipped with scraper means. The finely dispersed portion of the product stream is then recycled in this expensive and maintenance-intensive system, and, in any case, fractional desublimation is relatively infeasible.
Still another desublimation process is described in German Pat. No. 1,544,129 wherein the carrier gas containing the vapor-phase solid, and the cooling gas form two gaseous cylindrically-shaped flow patterns rotating coaxially in the same direction and moving axially in opposite directions, and wherein the desublimed solid is discharged from the desublimator together with an additional portion of the cooling gas. For this process, a very large quantity of cooling gas must be employed to prevent the gas-vapor mixture as well as the desublimed solid from coming into contact with the wall and in order to discharge the desublimed solid. Fractional desublimation is substantially impeded in this process because it is necessary for obtaining the desired flow pattern and movement of the gases to prevent them from complete intermingling. A large radial temperature gradient is present in the gaseous phase which extends from the temperature of the carrier gas to the temperature of the cooling gas. Therefore, normally the by-products are desublimed too.
In U.S. Pat. No. 4,281,518 a process and a device are described for separating a vaporous substance (e.g., sulfur dioxide vapor) from air by means of a liquefied inert gas, e.g., liquid nitrogen (the reference mentions using "liquid or cold inert gas" but apparently liquefied inert gas is the preferred coolant). The air-containing vapor and the liquefied inert gas are injected into the device through inlets which are aligned with one another. The substance to be separated hereby accumulates first as "snow", i.e., as a solid, and then is melted to a liquid. The temperature of the injected liquefied inert gas is always in the range of the condensation temperature of this gas; this temperature cannot be set at arbitrary values above the condensation temperature. Thus, the process for practical purposes is limited to the simultaneous separation of all substances which, down to the temperature of the injected liquefied inert gas, are converted from the vaporous condition to the liquid or solid condition. Separating of different vaporous substances during the separation process itself is practically impossible with this procedure since the desired vaporous substance and all by-products are simultaneously desublimated.
In U.S. Pat. No. 4,528,006 a device is described in which a vapor-gas mixture is cooled by means of an injected liquid, whereby the vaporous solid desublimates. The vapor-gas mixture and the liquid are injected into the device through inlets at the top of the device which are aligned with one another. In addition, liquid is sprayed into the device from the side whereby the nozzles for the liquid to be sprayed in from the side are attached to the device at various heights. The inside wall is designed so as to be permeable to gas and an additional gas constantly passes through this permeable wall so that the desublimated solid does not settle on the inside wall of the container. Since in this process, as well as that of U.S. Pat. No. 4,281,518, the vapor-gas mixture is cooled by a vaporizable liquid (e.g., water), in this process all substances that can be desublimated, which are contained in the vapor-gas mixture (in other words, the desired vaporous substance and all by-products) are simultaneously desublimated.